Depends on the situation, but IMHO you need to at least be willing to accept some responsibility in improving it, and it should affect how you deliver the criticism. (Even more kindly than usual.)

If you're in a position to criticise a finished solution proposal, you're probably part of the team and could have offered your input earlier, when it was potentially even requested. Maybe they knowingly took a shortcut to be able to move on to the other tasks and meet deadlines. Maybe they took on the task outside their core skillset because someone else dropped the ball. Maybe they have banged their head against it for days and simply cannot find a solution that works on all levels, already frustrated. Is there a superior or colleague they could have asked for assistance? Are you the superior?